Clones get a bad rap. In movie after movie, they are portrayed as somehow evil, from the clones of Hitler in The Boys from Brazil to the Imperial clone army in the Star Wars prequels*. In particular, it is often implied that clones are automata that work together for sinister ends.

Advertisement

The television series Orphan Black gets closer to the reality. It features clones with distinct personalities shaped by their life stories, who are no more evil than any of the other characters.

But maybe all the other science fiction authors have been taking notes from insects. Female pea aphids produce squadrons of genetically identical offspring, which swarm rapidly on their host plants. And if food runs short, the baby clones turn into parricidal vampires, draining the adults’ blood.

Munching army

Pea aphids are a major pest, damaging important crops like alfalfa, broad beans and – obviously – peas. Like many aphids, they have an unusual reproductive cycle. Males and females are only seen in early autumn, when they mate and produce a single egg that will outlast the winter.

When the egg hatches in spring, the young aphid grows up and starts giving birth to live young. She does not mate, but produces young from unfertilised eggs, a trick known as parthenogenesis. In effect, she makes an army of clones by virgin birth. Each generation then reproduces asexually until the nights draw in, at which point the aphids produce – again by virgin birth – a single generation of males and females to complete the cycle.

Each new clone army spreads over its host plant, and the insects use their piercing mouthparts to suck sap out of its leaves and stem. Their feeding mechanism resembles a hypodermic needle, explains Simon Leather of Harper Adams University in Newport, UK.

They also have symbiotic bacteria living in their bodies, which provide them with essential amino acids that they can’t get from the plants. Other bacteria help boost the aphids’ immune response, protecting them from disease. “These symbionts are almost part of the aphid,” says Leather.

Tiny green vampires

The pea aphids seem to be perfectly suited to a vegetarian lifestyle, but they don’t restrict themselves to eating plants. “Many years ago I was working on a pea aphid culture and suddenly felt a stinging sensation,” says Leather. When he looked down, he realised one of the insects was probing his wrist.

At the time, Leather thought it must have been confused because his hands smelled of pea plant. But several decades later, a student told him that the pea aphids in their lab were eating each other. Young aphids were climbing onto the backs of adults, probing them with their mouthparts and then sucking blood out of them.

Leather did some research and discovered a single prior report of aphid cannibalism, from 1968, which was based on second-hand information going back to 1930 (Nature, DOI&colon; 10.1038/218491a0).

Cannibal clones

To find out more, Leather and his colleagues placed groups of three pea aphids – a mother and two of her offspring – onto a broad bean plant (Vicia faba) leaves. Then they added a fourth, unrelated aphid, and watched their behaviour.

Cannibalism was common, and the juveniles did it far more often than the adults. Suspecting that this was a strategy to cope with food shortages, the team placed the insects on plants that had been damaged by giving them less water. Aphids on the more desiccated plants were more likely to resort to cannibalism than aphids on well-watered plants, suggesting that cannibalism was a response to lack of food. “In times of stress, this is a sort of survival mechanism,” says Leather.

The young aphids were choosy about who they attacked. They were more likely to target unrelated insects, which makes sense&colon; unrelated aphids are competitors and it might be advantageous to attack them regardless of the food situation. It also suggests that pea aphids can tell the difference between their own clones and unrelated insects, which nobody thought they could do.

Preying paradox

But there’s a weird twist. Despite being more likely to target unrelated aphids, actual episodes of blood sucking lasted longer if an insect was feasting on one of its clones.

That may seem paradoxical, but all the clones are genetically identical, so the individuals do not matter as much as the overall population. As a result, clones may be willing to let their fellows eat them because, by doing so, they are ensuring that at least some of the clones will survive to reproduce and pass on their shared genes. Unrelated aphids, by contrast, would probably resist being eaten.

This also helps explain why it is the young that prey on adults. “It’s more important for the juveniles to survive,” says Leather, so the adults may be prepared to sacrifice themselves. And from the youngsters’ point of view, it is all food. “Feeding on your mother is a good strategy.”